Electrochemical devices and fuel cell systems

ABSTRACT

Electrochemical devices including electrochemical pumps (ECPs) and fuel cell systems comprising a fuel cell and an ECP are disclosed. In particular, this electrochemical device can be an ECP that comprises an anode, a cathode and an anion exchange polymer separating the anode from the cathode. The ECP can be coupled to a hydroxide exchange membrane fuel cell (HEMFC) that is disclosed herein as a fuel cell system. These devices can be used in methods for removing carbon dioxide from air and for generating electricity.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. Non-Provisional applicationSer. No. 16/278,505 filed on Feb. 18, 2019, which claims the benefit ofU.S. Provisional Application No. 62/769,764 filed on Nov. 20, 2018. Theentire contents of the above applications are hereby incorporated byreference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with Government support under grantsDE-AR0000771 awarded by Advanced Research Projects Agency—Energy(ARPA-E) U.S. Department of Energy. The Government has certain rights inthe invention.

THE NAMES OF PARTIES TO A JOINT RESEARCH AGREEMENT

Not applicable.

REFERENCE TO A SEQUENCE LISTING, TABLE, OR COMPUTER PROGRAM LISTINGAPPENDIX SUBMITTED ON A COMPACT DISC AND AN INCORPORATION-BY-REFERENCEOF THE MATERIAL ON A COMPACT DISC

Not applicable.

FIELD OF THE INVENTION

Electrochemical devices, particularly electrochemical pumps (ECPs), andfuel cell systems comprising a hydroxide exchange membrane fuel cell(HEMFC) and an ECP are disclosed. These ECPs and systems can be used inmethods for removing carbon dioxide from air and for generatingelectricity by operation of a fuel cell with CO₂-containing air.

BACKGROUND OF THE INVENTION

Carbon dioxide (CO₂) is an acid gas present at roughly 400 ppm inatmospheric air. As an acid gas, CO₂ reacts with strong bases likehydroxide anions to form carbonate and bicarbonate anions.CO₂+OH⁻

HCO₃ ⁻  [1]HCO₃ ⁻+OH⁻

CO₃ ²⁻+H₂O  [2]Alkaline fuel cells and hydroxide exchange membrane fuel cells (HEMFCs)use hydroxide-conducting electrolytes and suffer significant efficiencylosses when exposed to CO₂. Liquid alkaline fuel cells suffer fromcarbonate precipitation, which clogs pores and can be fatal to the cell.HEMFCs have tethered cations that cannot form carbonate precipitates,but the efficiency of the HEMFC is reduced by concentration gradients ofcarbonate anions in the cell. When operating at steady state onCO₂-containing air, the anode consumes hydroxide and accumulatesbicarbonate until the local pH drops sufficiently low that bicarbonateis decomposed. The cell reaches a steady state where CO₂ is captured bythe cathode at the same rate it is released from the anode, and the pHgradient between anode and cathode typically causes a few hundred mV ofloss. The loss is typically 100-300 mV when the cathode gas contains 400ppm CO₂.

HEMFCs have potential cost advantages over the more common protonexchange membrane fuel cells (PEMFCs) due in large part to the improvedcorrosion resistance of many metals in alkaline electrolyte compared toacid. This enables nonprecious metal catalysts, especially at thecathode, and cheaper bipolar plate materials. However, as explainedabove, achieving good HEMFC performance and efficiency requires the useof air supply to the cathode that has a low concentration of CO₂.Therefore, a compact and low-cost device to generate an air streamhaving a low CO₂ concentration is important for a commercially viableHEMFC technology.

The current state-of-the-art for generating an air stream having a lowconcentration of carbon dioxide for an HEMFC is to use two or more bedsof regenerable polymer amine sorbents as disclosed in U.S. Pat. No.9,368,819. The beds are thermally regenerated, and a minimum of two bedsare required to provide continuous operation, so that one bed is onlinewhile the other bed is regenerating. This design is complex and bulky,and may not be suitable for transportation use or otherspace-constrained HEMFC applications.

Additionally, systems for removing carbon dioxide from a gas stream havemany applications outside of the field of HEMFCs. Additionalapplications include: CO₂ removal for metal-air batteries, breathing gaspurification for diving, submarine, or space applications; CO₂enrichment of greenhouses to accelerate plant growth; CO₂ capture fromflue gas or air for subsequent use or sequestration; and separation ofgases in industrial applications.

Therefore, a need exists for a more efficient and cost-effective deviceand method for removing carbon dioxide from carbon dioxide-containinggas that can be used with additional devices (e.g., fuel cells).

BRIEF SUMMARY OF THE INVENTION

The present disclosure is directed to fuel cell systems, electrochemicalpumps, and methods of using these to reduce the carbon dioxideconcentration in air and to generate electricity.

For example, the disclosure is directed to a fuel cell system comprisinga hydroxide exchange membrane fuel cell (HEMFC) and an electrochemicalpump (ECP) for separating carbon dioxide from a carbondioxide-containing gas, the ECP comprising a cell, the cell comprisingan anode, a cathode, and a membrane. The anode comprises an anodeelectrocatalyst for oxidizing a reagent to form protons or consumehydroxide ions; the cathode comprises a cathode electrocatalyst forreducing oxygen to form hydroxide ions; and the membrane is adjacent toand separates the anode and the cathode. The carbon dioxide-containinggas is supplied to the cathode and the carbon dioxide reacts with thehydroxide ions formed at the cathode to form bicarbonate ions, carbonateions, or bicarbonate and carbonate ions. The bicarbonate ions, carbonateions, or bicarbonate and carbonate ions are transported to the anodethrough the membrane; and the bicarbonate ions, carbonate ions, orbicarbonate and carbonate ions react at the anode to form carbon dioxideand water. The carbon dioxide-containing gas is air and after the airpasses through the cathode of the ECP to reduce the concentration of thecarbon dioxide, the air that has the reduced concentration of carbondioxide is directed to a cathode inlet of the HEMFC.

Additionally, the disclosure is directed to an internal-currentelectrochemical pump (iECP) for separating carbon dioxide from a carbondioxide-containing gas comprising a cell, the cell comprising an anode,a cathode, and a membrane. The anode comprises an anode electrocatalystfor oxidizing a reagent to form protons or consume hydroxide ions. Thecathode comprises a cathode electrocatalyst for reducing a reagent toform hydroxide ions. The membrane is adjacent to and separates the anodeand the cathode. The carbon dioxide-containing gas is supplied to thecathode and the carbon dioxide reacts with the hydroxide ions formed atthe cathode to form bicarbonate ions, carbonate ions, or bicarbonate andcarbonate ions. The bicarbonate ions, carbonate ions, or bicarbonate andcarbonate ions are transported to the anode through the membrane; andthe bicarbonate ions, carbonate ions, or bicarbonate and carbonate ionsreact at the anode to form carbon dioxide and water. The anode and thecathode are electronically connected through the membrane.

Further, the disclosure is directed to an electrochemical pump (ECP) forseparating carbon dioxide from air comprising a cell, the cellcomprising an anode, a cathode, and a membrane, and having air suppliedto the cathode and hydrogen supplied to the anode. The anode comprisesan anode electrocatalyst for oxidizing hydrogen to form protons orconsume hydroxide ions. The cathode comprises a cathode electrocatalystfor reducing oxygen in air to form hydroxide ions. The membrane isadjacent to and separates the anode and the cathode. The carbon dioxidein the air supplied to the cathode reacts with the hydroxide ions toform bicarbonate ions, carbonate ions, or bicarbonate and carbonateions. The bicarbonate ions, carbonate ions, or bicarbonate and carbonateions are transported to the anode through the membrane; and thebicarbonate ions, carbonate ions, or bicarbonate and carbonate ionsreact at the anode to form carbon dioxide and water.

Additionally, the disclosure is directed to a method for separatingcarbon dioxide from a carbon dioxide-containing gas or air comprisingsupplying the cathode of the electrochemical pump (ECP) of the fuel cellsystems described herein with the carbon dioxide-containing gas or airand supplying the anode of the ECP with a hydrogen-containing gas.

This disclosure is also directed to an electrochemical pump (ECP) forseparating carbon dioxide from a carbon dioxide-containing gascomprising a cell, the cell comprising a membrane and two electrodesthat are capable of acting as an anode or a cathode. The two electrodeseach independently comprises a charge-storage compound that reacts toform hydroxide when acting as cathode and reacts to consume hydroxide orproduce protons when acting as anode. The membrane is adjacent to andseparates the two electrodes. A carbon dioxide-containing gas iscontacted with the electrode acting as cathode and the carbon dioxidereacts with the hydroxide ions to form bicarbonate ions, carbonate ions,or bicarbonate and carbonate ions; the bicarbonate ions, carbonate ions,or bicarbonate and carbonate ions are transported to the electrodeserving as anode through the membrane; and the bicarbonate ions,carbonate ions, or bicarbonate and carbonate ions react at the electrodeacting as anode to form carbon dioxide and water. The ECP also comprisesmeans for reversing the direction of current flow and simultaneouslyalternating the electrode with which the carbon dioxide-containing gasis contacted, thereby allowing each electrode to act, in turn, as anodeand as cathode.

The disclose is further directed to a system comprising a metal-airbattery and the electrochemical pumps (ECPs) described herein, whereinthe carbon dioxide-containing gas is air and after the air is suppliedto the cathode of the ECP to reduce the concentration of the carbondioxide, the air having the reduced concentration of carbon dioxide isdirected to a cathode inlet of the metal-air battery.

Further disclosed is a battery system comprising a metal-air battery andan electrochemical pump (ECP) for separating carbon dioxide from acarbon dioxide-containing gas, the ECP comprising a cell, the cellcomprising an anode, a cathode, and a membrane. The anode comprises ananode electrocatalyst for oxidizing a reagent to form protons or consumehydroxide ions. The cathode comprises a cathode electrocatalyst forreducing oxygen to form hydroxide ions. The membrane is adjacent to andseparating the anode and the cathode. The carbon dioxide-containing gasis supplied to the cathode and the carbon dioxide reacts with thehydroxide ions formed at the cathode to form bicarbonate ions, carbonateions, or bicarbonate and carbonate ions. The bicarbonate ions, carbonateions, or bicarbonate and carbonate ions are transported to the anodethrough the membrane; and the bicarbonate ions, carbonate ions, orbicarbonate and carbonate ions react at the anode to form carbon dioxideand water. The carbon dioxide-containing gas is air and after the airpasses through the cathode of the ECP to reduce the concentration of thecarbon dioxide, the air having the reduced concentration of carbondioxide is directed to a cathode inlet of the metal-air battery.

Other objects and features will be in part apparent and in part pointedout hereinafter.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a schematic of the fuel cell system comprising a hydroxideexchange membrane fuel cell (HEMFC) and an electrochemical pump (ECP).Air is supplied to the cathode of the ECP where carbon dioxide reactswith electrochemically generated hydroxide. After the air passes throughthe cathode of the ECP, the concentration of CO₂ has been reduced, andthe air with reduced CO₂ concentration is fed to the cathode inlet ofthe HEMFC. For illustrative purposes, the system is drawn with hydrogenas the anode reagent in the ECP and with hydrogen being supplied fromthe purge stream of the HEMFC.

FIG. 2 is a schematic of the ECP operating with oxygen as the cathodereagent and hydrogen as the anode reagent, showing the electrochemicaland chemical reactions responsible for CO2 capture and release. Theelectronic current is shown as taking either an internal path (iECP) oran external path (eECP). The inset shows a stylistic representation ofone possible embodiment of the cathode or anode, which comprises anelectrocatalyst and an ionomer in a porous structure.

FIG. 3 and FIG. 4 are schematics of different planar hydrogen/air ECPconfigurations.

FIG. 5 is a schematic of a spiral wound module showing an example of apossible cell stacking configuration.

FIG. 6 is also a schematic of a spiral wound module showing an exampleof a possible configuration including the stacking of two cells andcurrent collectors for the stack.

FIG. 7 is a schematic of a spiral-wound module having an externalcurrent path and the module axial cross section is shown.

FIG. 8 is a schematic of a possible hydrogen inlet for the modulesdescribed herein.

FIG. 9 is a schematic of an iECP and the cell stacking of the module isdetailed.

FIG. 10 is a schematic of a spiral wound module having the cell stacksdetails in FIG. 9 .

FIG. 11 is schematic of a hollow fiber having an iECP fabricated intothe shell.

FIG. 12 is a schematic of a module comprising multiple hollow fibers asrepresented in FIG. 11 .

FIG. 13 is a graph of the modeled concentration profiles of anions inthe membrane electrode assembly (MEA) at 20 mA/cm². The cell temperatureis 70° C. and the gases supplied to anode gas flow layer and cathode gasflow layer are hydrogen with 100,000 ppm CO₂ and air with 400 ppm CO₂respectively, both at 2 bar.

FIG. 14A is a graph of the modeled anion concentration profiles (at 20mA/cm²) through the thickness of the MEA at a location corresponding tothe cathode outlet at 99.9% CO₂ removal. The cell temperature is 70° C.and the gases supplied to anode gas flow layer and cathode gas flowlayer are hydrogen with 100,000 ppm CO₂ and air with 0.4 ppm CO₂respectively, both at 2 bar.

FIG. 14B is a graph of the modeled CO₂ reaction rate profile. Positiverates signify CO₂ capture and negative rates signify CO₂ release. Thecell temperature is 70° C. and the gases supplied to anode gas flowlayer and cathode gas flow layer are hydrogen with 100,000 ppm CO₂ andair with 0.4 ppm CO₂ respectively, both at 2 bar.

FIGS. 15A and 15B are graphs of the measured cathode outlet CO₂concentration from 25 cm² ECP (cell #2) operating in H₂/air mode with arange of air flow rates. FIG. 15A shows results at a constant currentdensity of 10 mA/cm². FIG. 15B shows results at a constant currentdensity of 20 mA/cm². The anode flow rate is 50 sccm, relative humidity(RH) is 80%, and the outlet pressure is ambient. CO₂ concentrations wereaveraged over the final 30 minutes of a 60 minute hold.

FIGS. 16A and 16B are graphs of a measured CO₂ ECP performance at 70°C., 80% RH, 20 mA/cm² for low-loading cell with and without ionomerinterlayers and a conventional high-loading cell. The high-loading cellwas tested at 80° C., 90% RH, and 5 cm² active area. FIG. 16A shows thecathode outlet CO₂ concentration as a function of air flow rate. The 5cm² MEA flowrate (high loading) was scaled to a 25 cm² equivalent forcomparison. FIG. 16B shows the calculated average mass transportresistance as a function of outlet CO₂ concentration. Results below 1ppm are excluded due to excessive measurement uncertainty. Allmeasurements were averaged over final 30 min of 60 minute hold.

FIG. 17 is a graph of the measured performance of an iECP operating withhydrogen as the anode reagent and oxygen as the cathode reagent. CO₂concentration measured at the anode and cathode outlets with gas flow at0.1 L min⁻¹, 90% relative humidity, and ambient pressure. The cathodefeed gas was air with 350 ppm CO₂. Anode gas was N₂ or H₂ as indicated(controlling cell output). When N₂ was used as the anode gas, nosignificant current was generated in the cell and very little CO₂transport took place. When H₂ was used as the anode gas, the resultingcurrent flow caused electrochemical pumping of CO₂, which was capable of“uphill” CO₂ pumping. “Uphill” CO₂ pumping means that the cathode gasstream from which CO₂ was transported had a lower CO₂ concentration thanthe anode gas stream to which CO₂ was transported. Such transport cannotbe driven by the concentration gradient alone, which points in the wrongdirection, and must be the result of electrochemical pumping.

Corresponding reference characters indicate corresponding partsthroughout the drawings.

DETAILED DESCRIPTION OF THE INVENTION

The present disclosure is directed to an electrochemical pump (ECP) forseparating carbon dioxide from a carbon dioxide-containing gas. This ECPcomprises an anode, a cathode and an anion exchange polymer membranebeing adjacent to and separating the anode and the cathode. The ECP canbe coupled to a hydroxide exchange membrane fuel cell (HEMFC) to form asystem that is disclosed herein as a fuel cell system. A schematic ofone example of the fuel cell system is represented in FIG. 1 . The fuelcell system can be used in methods to generate electricity.

The ECPs described herein can be used to remove CO₂ from a gas streamusing a membrane electrode assembly (MEA), where hydroxide is generatedelectrochemically at the cathode, and hydroxide is consumed, or protonsgenerated, electrochemically at the anode. One embodiment of the ECP isillustrated in FIG. 2 . CO₂ is captured at the cathode by reaction withhydroxide according to Equation 1. Carbonate and bicarbonate anions aredriven by the electric field to the anode, where CO₂ is released throughthe overall reactionHCO₃ ⁻+H⁺

H₂O+CO₂,  [3]that may occur in two steps where proton transfer may occur before orafter CO₂ release.

Many anode and cathode reactions are possible to generate protons andhydroxide, respectively. Preferred anode reactions include the hydrogenoxidation reaction (HOR),H₂

2H⁺+2e ⁻, or,H₂+2OH−

2H₂O+2e ⁻  [4a]ammonia oxidation reaction (AOR),2NH₃

N₂+6H⁺+6e ⁻, or2NH₃+6OH—

N₂+3H₂O+6e ⁻  [5]oxygen evolution reaction (OER),2H₂O

O₂+4H⁺+4e ⁻, or,4OH−

O₂+2H₂O+4e ⁻  [6]and nickel hydroxide oxidation reaction (NiOR),OH⁻+Ni(OH)₂

NiOOH+H₂O+e ⁻.  [7]Preferred cathode reactions include hydrogen evolution reaction (HER),2H₂O+2e ⁻

H₂+2OH⁻, [8]oxygen reduction reaction (ORR),O₂+H₂O+4e ⁻

4OH⁻,  [9]and nickel oxyhydroxide reduction reaction (NiRR),NiOOH+H₂O+e ⁻

Ni(OH)₂+OH⁻.  [10]Using NiOR and NiRR (Equations 7 and 10) or other charge storageelectrode reactions, a nearly pure CO₂ product stream can be recovered.Continuous operation can be achieved by periodically reversing the cellcurrent and simultaneously switching the gas connections when theelectrodes become fully or close to fully charged/discharged.

Also disclosed is a fuel cell system comprising a HEMFC and an ECP forseparating carbon dioxide from a carbon dioxide-containing gascomprising a cell, the cell comprising an anode, a cathode, and amembrane. The anode comprises an anode electrocatalyst for oxidizing areagent to form protons or consume hydroxide ions; the cathode comprisesa cathode electrocatalyst for reducing oxygen to form hydroxide ions;and the membrane being adjacent to and separating the anode and thecathode. The carbon dioxide-containing gas is supplied to the cathode ofthe ECP and the carbon dioxide reacts with the hydroxide ions formed atthe cathode to form bicarbonate ions, carbonate ions, or bicarbonate andcarbonate ions; the bicarbonate ions, carbonate ions, or bicarbonate andcarbonate ions are transported to the anode through the membrane; andthe bicarbonate ions, carbonate ions, or bicarbonate and carbonate ionsreact at the anode to form carbon dioxide and water. The carbondioxide-containing gas is typically air and after the air is supplied tothe cathode of the ECP to reduce the concentration of the carbondioxide, the air having the reduced concentration of carbon dioxide isdirected to a cathode inlet of the HEMFC.

A schematic of the fuel cell system is represented in FIG. 1 .

The fuel cell system described herein can have the carbondioxide-containing gas supplied to the HEMFC cathode contain less thanabout 20 ppm, 18 ppm, 16 ppm, 15 ppm, 12 ppm, 10 ppm, 8 ppm, 6 ppm, 5ppm, 4 ppm, 3 ppm, or 2 ppm carbon dioxide with these reduced levelsachieved by reaction of CO₂ with the hydroxide ions at the cathode ofthe ECP.

Additionally, the fuel cell system described herein can have the reagentoxidized at the anode electrocatalyst of the ECP be hydrogen and thehydrogen consumed by the ECP for separating carbon dioxide from air isless than about 5%, 4%, 3%, or 2% of the hydrogen consumed by the HEMFC.

For the application of generating CO₂-free air for an HEMFC, the bestchoices of electrode processes in the ECP are HOR (Equation 4) at theanode and ORR (Equation 9) at the cathode, because oxygen is availablein the air stream to be purified and hydrogen can be purged from thestack to supply the anode. An additional advantage of these reactions isthat they generate enough electromotive force to power the cell, withoutrequiring an external power supply.

The core component of the ECP is the MEA, which comprises a membranewith an electrode on each side. Both electrodes comprise anelectrocatalyst and an anion exchange polymer with porosity sufficientto enable gas transport. The electrodes are conductive for bothelectrons and anions. The membrane comprises an anion exchange polymerand may optionally include reinforcement polymers or electron-conductingadditives. If the membrane conducts both electrons and anions, then noexternal electrical connections are needed, and the MEA can be used inany module configuration, similar to non-electrochemical membranes. AnECP with a membrane that conducts both electrons and anions is referredto herein as an internal-current electrochemical pump (iECP). If themembrane conducts anions only, not electrons, then an external currentpath must be included in the module. An ECP that requires an externalcurrent path is referred to herein as an external-currentelectrochemical pump (eECP).

The disclosure is also directed to an iECP for separating carbon dioxidefrom a carbon dioxide-containing gas that has the anode and cathodeelectronically connected through the anion exchange membrane. When apotential difference appears across this type of cell, both the ionicand electronic currents generated pass through the membrane. This iECPcomprises a cell, the cell comprises an anode, a cathode, and amembrane. The anode comprises an anode electrocatalyst for oxidizing areagent to form protons or consume hydroxide ions; the cathodecomprising a cathode electrocatalyst for reducing a reagent to formhydroxide ions; and the membrane being adjacent to and separating theanode and the cathode. The carbon dioxide-containing gas is supplied tothe cathode and the carbon dioxide reacts with the hydroxide ions formedat the cathode to form bicarbonate ions, carbonate ions, or bicarbonateand carbonate ions; the bicarbonate ions, carbonate ions, or bicarbonateand carbonate ions are transported to the anode through the membrane;and the bicarbonate ions, carbonate ions, or bicarbonate and carbonateions react at the anode to form carbon dioxide and water; and the anodeand the cathode are electronically connected through the membrane.

A schematic of the ECP having either an internal current path (iECP asdescribed immediately above) or an external current path (eECP) isrepresented in FIG. 2 .

The iECP disclosed herein can have the membrane comprise an anionexchange polymer and an electronically-conductive material or anelectronically-conductive anion exchange polymer.

The iECP can have the anion exchange polymer comprise quaternaryammonium or imidazolium groups and a polymer backbone not having ethergroups.

Preferably, the iECP described herein can have the anion exchangepolymer comprises poly(aryl piperidinium), alkylammonium-functionalizedpoly(aryl alkylene), substituted-imidazolium-functionalized poly(arylalkylene), alkylammonium-functionalized poly(styrene),substituted-imidazolium-functionalized poly(styrene),alkylammonium-functionalized poly(styrene-co-divinylbenzene),substituted-imidazolium-functionalized poly(styrene-co-divinylbenzene),alkylammonium-functionalizedpoly(styrene-block-ethylene-co-butadiene-block-styrene),substituted-imidazolium-functionalized,poly(styrene-block-ethylene-co-butadiene-block-styrene),alkylammonium-functionalized poly(ethylene),substituted-imidazolium-functionalized poly(ethylene),alkylammonium-functionalized poly(tetrafluoroethylene),substituted-imidazolium-functionalized poly(tetrafluoroethylene),alkylammonium-functionalized poly(ethylene-co-tetrafluoroethylene),substituted-imidazolium-functionalizedpoly(ethylene-co-tetrafluoroethylene), polyethyleneimine, poly(diallylammonium), or a combination thereof.

The iECP can have the electronically-conductive material comprisecarbon, nickel, stainless steel, silver, an electronically conductivepolymer, or a combination thereof. Additionally, the electronicallyconductive material comprises nanowires or nanotubes.

These electronically-conductive materials that are metals can also bealloys with additional metals.

The iECP can comprise one or more cells that are arranged in aconfiguration of a hollow fiber.

The hollow fiber would have the cathode on the inside (lumen) and theanode on the outside (shell). The CO₂-containing gas would pass throughthe lumen, and the anode reactant would be fed to the shell side.

A module could be constructed with one or more fibers encased in acylindrical housing, with the fibers potted in a sealing compound(typically epoxy) forming a bulkhead near each end. The lumen is influid communication with the ends of the module beyond the bulkheads,while the shell space is between the two bulkheads and isolated from theends. The inlet and outlet for the CO₂-containing gas are the two ends.The inlet and outlet for anode reactant and separated CO₂ are betweenthe two bulkheads. Countercurrent flow would be advantageous, but notstrictly required.

One way this hollow fiber configuration can be arranged is representedin the schematic of FIGS. 11 and 12 .

In the hollow-fiber architecture, the lumen of the fiber is the cathodeside and the shell of the fiber is the anode side. The hollow fibers arecombined in a bundle and placed in a cylindrical housing with the endspotted in epoxy and cut open. Ports are added to the housing above andbelow each of the epoxy plugs to give gas access to the lumen and shellside of the fibers. The hollow fiber can be made with severalconfigurations, and as disclosed above, FIGS. 11 and 12 show particularexamples that fall within this type of configuration.

The iECP can comprise one or more additional cells and the cells cancontain an anode gas flow layer adjacent to one or two anodes, the anodeadjacent to the membrane, the membrane adjacent to the anode and thecathode, a cathode gas flow layer adjacent to one or two cathodes, theconfiguration being represented as follows:[-AG-A-M-C-CG-C-M-A-]wherein AG is the anode gas flow layer, A is the anode, M is themembrane, C is the cathode, and CG is the cathode gas flow layer.

More specifically, for the iECP, both planar and spiral-woundarchitectures are possible, as well as a hollow fiber architecture.There is no requirement to electrically connect individual cells, whichexpands the possibilities. For the planar and spiral-woundconfigurations, the cells do not require bipolar plates, but instead canbe arranged in a pattern of CMA|AG|AMC|CG|CMA|AG|AMC|CG| . . . , whereCMA is an MEA with cathode on the left and anode on the right, AMC is anMEA with anode on the left and cathode on the right, CG is a cathode gasflow layer, and AG is an anode gas flow layer. The spiral-wound moduleuses one or more leaves of CMA|AG|AMC|CG and wraps them in a spiralpattern so that CG of one wrap or leaf contacts CMA of the next wrap orleaf.

This configuration provides the advantage that adjacent cells can sharea cathode gas flow layer or an anode gas flow layer. This configurationis enabled by iECP design. A schematic of this configuration isrepresented in FIG. 10 .

The iECP described herein can also be incorporated into a fuel cellsystem comprising a HEMFC. The carbon dioxide-containing gas is air andafter the air is passed through the cathode of the iECP to reduce theconcentration of carbon dioxide, the air having the reducedconcentration of carbon dioxide is directed from the cathode exhaust ofthe iECP to the cathode inlet of the HEMFC.

Further, the disclosure is directed to an ECP for separating carbondioxide from air that has hydrogen directed to the anode and airdirected to the cathode and uses an anion exchange polymer membrane aselectrolyte placed between and adjacent to the anode and the cathode.

The ECP comprises a cell, and the cell comprises an anode, a cathode,and a membrane. The cell has air supplied to the cathode and hydrogensupplied to the anode. The anode comprises an anode electrocatalyst foroxidizing hydrogen to form protons or consume hydroxide ions; thecathode comprises a cathode electrocatalyst for reducing oxygen in airto form hydroxide ions; and the membrane is adjacent to and separatesthe anode and the cathode. The carbon dioxide in the air supplied to thecathode reacts with the hydroxide ions to form bicarbonate ions,carbonate ions, or bicarbonate and carbonate ions; the bicarbonate ions,carbonate ions, or bicarbonate and carbonate ions are transported to theanode through the membrane; and the bicarbonate ions, carbonate ions, orbicarbonate and carbonate ions react at the anode to form carbon dioxideand water.

The general schematic of this ECP is represented in FIG. 2 . A schematicof some planar hydrogen/air ECP configurations are represented in FIGS.3 and 4 .

The fuel cell system comprising the HEMFC and ECP or the ECP describedherein can have the reagent oxidized by the anode electrocatalyst behydrogen, ammonia, hydrazine, methanol, ethanol, urea, or a combinationthereof. Preferably, the reagent oxidized at the anode of the ECPcomprises hydrogen or ammonia. More preferably, the reagent oxidized atthe anode electrocatalyst comprises hydrogen.

The HEMFC and ECP fuel cell system or the ECP described herein can havethe reagent reduced at the cathode electrocatalyst of the ECP comprisesoxygen, hydrogen peroxide, or a combination thereof. Preferably, thereagent at the cathode comprises oxygen.

The fuel cell system comprising a HEMFC and ECP or the ECP describedherein can have the anode electrocatalyst of the ECP include platinum, aplatinum alloy, carbon-supported platinum, a carbon-supported platinumalloy, nickel, a nickel alloy, carbon-supported nickel, acarbon-supported nickel alloy, ruthenium, a ruthenium alloy,carbon-supported ruthenium, a carbon-supported ruthenium alloy, iridium,a iridium alloy, carbon-supported iridium, a carbon-supported iridiumalloy, palladium, a palladium alloy, carbon-supported palladium, acarbon-supported palladium alloy, or a combination thereof. Preferably,the anode electrocatalyst comprises a carbon-supported platinum.

The HEMFC and ECP fuel cell system or the ECP described herein can havethe cathode electrocatalyst of the ECP include silver, a silver alloy,carbon-supported silver, a carbon-supported silver alloy, platinum, aplatinum alloy, carbon-supported platinum, a carbon-supported platinumalloy, palladium, a palladium alloy, carbon-supported palladium, acarbon-supported palladium alloy, manganese oxide, a carbon-supportedmanganese oxide, cobalt oxide, a carbon-supported cobalt oxide,heteroatom-doped carbon (X—C, where X comprises one or more of N, C, B,P, S, Se, or O), metal-heteroatom-carbon (M-X—C, where X comprises oneor more of N, C, B, P, S, Se, or O, and M comprises one or more of Fe,Ce, Cr, Cu, Co, Mo, Ni, Ru, Pd, Pt, Ir, Rh, Os, Ag, Au, Re, Ta, Ti, V,W, Mn, Zn, Sn, Sb, In, Ga, Bi, Pb, or Zr), a perovskite (ABX₃ where Acomprises one or more of Ca, Sr, Ba, Sc, Y, La, Ce, Zr, Cu, Zn, Sb, Bi,B comprises one or more of Al, Ti, Mn, Fe, Co Ni, W, Pd, and X comprisesone or more of O, Se, S), a carbon-supported perovskite (ABX₃ where Acomprises one or more of Ca, Sr, Ba, Sc, Y, La, Ce, Zr, Cu, Zn, Sb, Bi,B comprises one or more of Al, Ti, Mn, Fe, Co Ni, W, Pd, and X comprisesone or more of O, Se, S), or a combination thereof. Preferably, thecathode electrocatalyst comprises silver.

The HEMFC and ECP fuel cell system or the ECP described herein can havethe membrane of the ECP comprise an anion exchange polymer.

The anion exchange polymer can comprise poly(arylpiperidinium),alkylammonium-functionalized poly(aryl alkylene),substituted-imidazolium-functionalized poly(aryl alkylene),alkylammonium-functionalized poly(styrene),substituted-imidazolium-functionalized poly(styrene),alkylammonium-functionalized poly(styrene-co-divinylbenzene),substituted-imidazolium-functionalized poly(styrene-co-divinylbenzene),alkylammonium-functionalizedpoly(styrene-block-ethylene-co-butadiene-block-styrene),substituted-imidazolium-functionalized,poly(styrene-block-ethylene-co-butadiene-block-styrene),alkylammonium-functionalized poly(ethylene),substituted-imidazolium-functionalized poly(ethylene),alkylammonium-functionalized poly(tetrafluoroethylene),substituted-imidazolium-functionalized poly(tetrafluoroethylene),alkylammonium-functionalized poly(ethylene-co-tetrafluoroethylene),substituted-imidazolium-functionalizedpoly(ethylene-co-tetrafluoroethylene), polyethyleneimine, poly(diallylammonium), or a combination thereof. Preferably, the anion exchangepolymer comprises poly(arylpiperidinium).

The ECP MEA can be combined with gas flow layers, optional gas diffusionlayers, and optional separators to create a cell of the ECP. One or morecells are packaged with gas manifolds, housing, and seals to make an ECPmodule. The ECP module is combined with a controller to make a completeECP. Finally, and depending on the application, the ECP can beintegrated with an HEMFC stack and other balance-of-system components tomake an air-fed ECP-HEMFC system. An example of an air-fed ECP-HEMFCsystem is shown in FIG. 1 .

The eECP as well as an eECP used in the fuel cell system describedherein, can have a current be supplied to it by an external powersource, or it can have a current drawn by a load if the electromotiveforce of the electrochemical cell is sufficient to drive the current.

The fuel cell system comprising a HEMFC and ECP or the ECP describedherein can have the ECP have one or more additional cells.

The ECP as well as the ECP in the fuel cell system described herein canhave one or more additional cells electrically connected in series.

For the eECP, several cell and module configurations are possible. Themodule architecture can be planar or spiral-wound. Planar modulescomprise a stack of planar cells, with manifolds incorporated into theborder region outside of the active area to distribute gases to eachcell. Cells may be separated by bipolar plates that incorporate flowchannels or the cells may be separated by planar bipolar plates withconductive mesh feed spacers used to provide flow pathways. This type ofconfiguration is represented in FIG. 3 .

The HEMFC and ECP fuel cell system or the ECP described herein can havethe cells be electrically connected in series by an electricallyconductive bipolar plate.

The ECP as well as the ECP in the fuel cell system described herein canhave each cell further comprise an anode gas flow layer and a cathodegas flow layer.

The ECP as well as the ECP in the fuel cell system described herein canhave the anode gas flow layer, the cathode gas flow layer, or the anodegas flow layer and the cathode gas flow layer comprise a flow field ofone or more flow channels alternated with a conductive material toprovide an electrical connection between the anode, the cathode, or theanode and cathode and the bipolar plate.

A typical bipolar plate is a thin sheet of stainless steel. One side iselectrically connected to the anode, and the other side is electricallyconnected to the cathode of the adjacent cell.

The bipolar plate may be integrated with one or both adjacent gas flowlayers. In this case, the bipolar plate is typically stamped to createflow channels on both sides (a corrugated structure).

The ECP as well as the ECP in the fuel cell system described herein canhave two or more flow channels of the cathode gas flow layer or two ormore flow channels of the anode gas flow layer are arranged in asubstantially parallel configuration.

The ECP as well as the ECP in the fuel cell system described herein canhave two or more flow channels of the cathode gas flow layer or two ormore flow channels of the anode gas flow layer arranged in aninterdigitated configuration.

The ECP as well as the ECP in the fuel cell system described herein canhave the bipolar plate integrated with an adjacent anode gas flow layeror an adjacent cathode gas flow layer.

The ECP as well as the ECP in the fuel cell system described herein canhave the bipolar plate integrated with the adjacent anode gas flow layerand the adjacent cathode gas flow layer.

The ECP as well as the ECP in the fuel cell system described herein canhave the anode gas flow layer, the cathode gas flow layer, or the anodegas flow layer, and the cathode gas flow layer comprise an electricallyconductive feed spacer.

The fuel cell system comprising a HEMFC and an ECP or the ECP describedherein can have the electrically conductive feed spacer made of a meshmade of nickel, a nickel alloy, stainless steel, anelectrically-conductive polymer, carbon fiber paper, or a combinationthereof.

The ECP as well as the ECP in the fuel cell system described herein canhave the electrically conductive feed spacer comprising a perforatedmetal sheet.

The ECP as well as the ECP in the fuel cell system described herein canhave the cells substantially planar and arranged in a stack.

The HEMFC and ECP fuel cell system or the ECP described herein can havethe cells be in a stack and formed around an inner tube to form a spiralstack.

The ECP as well as the ECP in the fuel cell system described herein canhave each cell comprise a cathode gas flow layer and the cathode gasflow layer in fluid connection with an axial end of a spiral stack.

The ECP as well as the ECP in the fuel cell system described herein canhave each cell comprise an anode gas flow layer and the anode gas flowlayer is in fluid connection with the inner surface of the tube and theouter radial surface of the spiral stack. The air can enter and leave onthe axial ends of the spiral stack as shown in FIGS. 7 and 10 .

A spiral wound module is represented in FIG. 5 with particular cellstacking detail. An additional configuration for cell stacking isrepresented in FIG. 6 that details the stacking of two cells andincludes current collectors for the stack.

FIG. 7 represents an example of a spiral-wound module having an externalcurrent path and the module axial cross section is shown. A person ofordinary skill in the art would have known that fewer or more cellscould be stacked in series before winding up the module.

Also, the inner tube could be divided to serve as both the hydrogeninlet and the outlet for the carbon dioxide-rich hydrogen. For example,the HEMFC and ECP fuel cell system or the ECP described herein can havethe cell comprises an anode gas flow layer and the anode gas flow layeris in fluid connection with a first manifold and a second manifold inthe inner tube. Further, the anode gas flow layer can comprise aflow-directing element that causes gas to flow from the first manifoldin the inner tube, outward through one portion of the anode gas flowlayer, and then inward through a second portion of the anode gas flowlayer to the second manifold in the inner tube. This configuration foris shown in detail in FIG. 8 .

The spiral wound module configuration comprises a stack of several cellsthat are rolled into a spiral-wound cylindrical module format. Each cellcomprises a MEA sandwiched between anode and cathode feed spacers, abipolar plate made of metal foil, and gaskets that seal the edges of thecell, providing an axial flow pathway on the cathode side and a radialflow pathway on the anode side. There are two configurations for theanode inlet and outlet. The spiral-wound module is made by wrapping thecell stack around an inner tube and is inserted into a cylindricalhousing. The anode inlet and outlet ports can be at the inner and outerradial ends of the spiral, in either order. Alternatively, the anodeinlet and outlet ports can both be the inner tube, with a bulkhead inthe center that separates the two ports. Then, flow-directing elementscan be added to the anode feed spacer to direct gas in a U-pattern outto the end of the leaf and back inward. The simplest flow-directingelement would be a sealant bead or gasket applied in a line from thebulkhead out to nearly the end of the leaf, which the gas must flowaround. However, there could be some stagnant zones near the outercorners of the leaf, so it might be better to use multiple gaskets orsealant beads to make nesting U-shaped flow channels.

For the iECP, the spiral wound module can have cell stacking representedin FIG. 9 and a spiral wound module as represented in FIG. 10 . Thespiral wound module could also have the hydrogen inlet as represented inFIG. 8 .

The HEMFC and ECP fuel cell system or the ECP described herein can havethe cell pitch of the ECP is less than about 2 mm, less than about 1.5mm, or less than about 1 mm.

The iECP described herein can have air as the carbon dioxide-containinggas.

The ECP as well as the ECP in the fuel cell system described herein canhave the membrane area/air flow rate ratio be less than or equal to 50cm² standard liter per minute (SLPM) at 1 atmosphere.

The ECP as well as the ECP in the fuel cell system described herein canhave the cell volume/air flow rate ratio be less than or equal to 10cm³/SLPM.

Also disclosed is a method for separating carbon dioxide from air oranother carbon dioxide-containing gas that comprises supplying thecathode of the ECP described herein or to the ECP in the HEMFC fuel cellsystem with the carbon dioxide-containing gas and supplying the anode ofthe ECP with a hydrogen-containing gas.

The method can further comprise passing a current, I_(cell),proportional to N_(CO2), the number of moles of CO₂ entering the cathodeinlet per second per cell, with I_(cell) defined by:I_(cell)=nF N_(CO2)where n is a number in the range 2-50 and F is the Faraday constant.Operating the ECP within this range of n, can achieve practicallycomplete removal of CO₂ from the air stream while minimizing use ofhydrogen. For the methods described herein, the carbondioxide-containing gas can be air.

Further, for the methods described herein, the carbon dioxide-containinggas can be a flue gas.

Additionally, the carbon dioxide in the ECP anode outlet stream can becollected. When the carbon dioxide is collected as a mixture withhydrogen, the hydrogen:carbon dioxide ratio can be between about 1:1 andabout 4:1.

The hydrogen and carbon dioxide mixture (e.g., synthesis gas) can be fedto a downstream reactor, where the desired ratio depends on thedownstream product. For example, the hydrogen:carbon dioxide ratio canbe about 4:1 for the Sabatier process (methane), about 3:1 for methanol,about 2:1 for the Fischer-Tropsch process, or about 2:1 for the Boschreaction (for oxygen recycling on spacecraft (e.g., CO₂+2 H₂=C+2H₂O).

Further disclosed is an ECP for separating carbon dioxide from a carbondioxide-containing gas comprising a cell, the cell comprising a membraneand two electrodes that are each capable of acting as an anode or acathode; the two electrodes each independently comprising acharge-storage compound that reacts to form hydroxide when serving ascathode and reacts to consume hydroxide or produce protons when servingas anode; the membrane being adjacent to and separating the twoelectrodes; wherein a carbon dioxide-containing gas is contacted withthe electrode serving as cathode and the carbon dioxide reacts with thehydroxide ions to form bicarbonate ions, carbonate ions, or bicarbonateand carbonate ions; the bicarbonate ions, carbonate ions, or bicarbonateand carbonate ions are transported to the electrode serving as anodethrough the membrane; and the bicarbonate ions, carbonate ions, orbicarbonate and carbonate ions react at the electrode serving as anodeto form carbon dioxide and water; wherein the ECP also comprises meansfor reversing the direction of current flow and simultaneouslyalternating the electrode with which the carbon dioxide-containing gasis contacted, thereby allowing each electrode to serve, in turn, asanode and as cathode.

The ECP described above can have one or both electrodes comprise a metaloxide, a metal hydroxide, a metal oxyhydroxide, or a hydrogen storagealloy. The metal oxyhydroxide can comprise nickel oxyhydroxide. Themetal oxide can comprise manganese dioxide. The hydrogen storage alloycan comprise a lanthanum nickel hydride.

Also disclosed in a method for separating carbon dioxide from a carbondioxide-containing gas comprising supplying the cathode of theelectrochemical pump (ECP) as described having one or both electrodescomprising a metal oxide, a metal hydroxide, a metal oxyhydroxide, or ahydrogen storage alloy with the carbon dioxide-containing gas.

Preferably, for this method described immediately above, the carbondioxide-containing gas is a flue gas.

The current in the ECP is supplied by a power supply, and the powersupply can directly reverse its output current or a dual pole dual throwswitch/relay can be used to reverse the connections between theterminals of the ECP and the terminals of the power supply.

For gas flows, four way valves at inlet and outlet are required. Forexample, the gas flows can be arranged so that in Mode A, Electrode 1has the CO₂-containing gas flowing in, and the CO₂-depleted gas flowingout and Electrode 2 has the sweep gas flowing in (optional), and theCO₂-enriched gas flowing out. In Mode B, Electrode 1 has the sweep gasflowing in (optional), and the CO₂-enriched gas flowing out andElectrode 2 has the CO₂-containing gas flowing in, and the CO₂-depletedgas flowing out.

Further disclosed is a battery system comprising a metal-air battery andan electrochemical pump (ECP) for separating carbon dioxide from acarbon dioxide-containing gas, the ECP comprising a cell, the cellcomprising an anode, a cathode, and a membrane. The anode comprises ananode electrocatalyst for oxidizing a reagent to form protons or consumehydroxide ions. The cathode comprises a cathode electrocatalyst forreducing oxygen to form hydroxide ions. The membrane is adjacent to andseparating the anode and the cathode. The carbon dioxide-containing gasis supplied to the cathode and the carbon dioxide reacts with thehydroxide ions formed at the cathode to form bicarbonate ions, carbonateions, or bicarbonate and carbonate ions. The bicarbonate ions, carbonateions, or bicarbonate and carbonate ions are transported to the anodethrough the membrane; and the bicarbonate ions, carbonate ions, orbicarbonate and carbonate ions react at the anode to form carbon dioxideand water. The carbon dioxide-containing gas is air and after the airpasses through the cathode of the ECP to reduce the concentration of thecarbon dioxide, the air having the reduced concentration of carbondioxide is directed to a cathode inlet of the metal-air battery.

When considering the following equation:I_(cell)=nF N_(CO2),It is desirable to operate at low values of n to lower energyconsumption, but the rate of capture of carbon dioxide declines at lowvalues of n. The reason this occurs is because the hydroxide:carbonateratio in the cathode is decreasing (i.e., decreasing hydroxidegeneration). Hydroxide is the active agent for CO₂ capture, so a lowerhydroxide concentration will reduce the rate of capture. As thehydroxide concentration decreases, the kinetics of CO₂ capture decreasebefore the equilibrium partial pressure of CO₂ becomes significant,which means that the same high fraction of CO2 capture is possible, butthe required ECP area is larger. With an appropriate catalyst, the CO₂capture rate can be maintained at low values of n (e.g. n=2-10),reducing energy (e.g., hydrogen) consumption without requiring a largerECP.

Primary, secondary, and tertiary amines are active for CO₂ capture. Withtertiary amines, CO₂ and water react to form a tertiary ammoniumbicarbonate (R₃NH⁺HCO₃ ⁻). If the tertiary amine is incorporated intothe ionomer (physically or chemically), the bicarbonate can be handedoff to the ionomer, and the ammonium can be rapidly neutralized byhydroxide so that it is active for CO₂ capture again. The key advantageis that the concentration of tertiary amine can be very high, even ifsubstantial carbonate buildup has occurred, and only a small amount ofhydroxide is present.

Primary and secondary amines can form bicarbonate salts, but ammoniumcarbamates —R₂HNH⁺ R₂HNCOO⁻ are predominantly formed.

As to the construction, one method is to incorporate branchedpolyethyleneimine into the cathode structure together with the ionomer.A second method is to use an ionomer with a combination of quaternaryammonium and primary-tertiary amines.

Similar to the rationale for the CO₂ hydration catalyst, it is desirableto improve performance at low values of n (low current density). The CO₂capture rate decreases when carbonate builds up at the cathode andlowers the hydroxide concentration. For high enough values of membraneresistance, the ratio of carbonate to hydroxide in the cathode will bedetermined by the ratio of their rates of generation and theirelectrochemical mobility in the ionomer. At this limit, anion transportthrough the ionomer is dominated by migration.

However, for lower values of membrane resistance, the potential gradientis smaller, and diffusion plays a role. The concentration of carbonateand bicarbonate are very high near the anode and diffusion will pushhydroxide towards the anode and push carbonate back towards the cathode,causing more carbonate buildup, and reducing the CO₂ capture rate.

If the membrane resistance is too high, there will not be a sufficientelectromotive force to drive the current. Ideally, the Ohmic (iR) lossis maintained between 10 mV and 300 mV. If the design current were 5-30mA/cm², the membrane resistance can be a minimum of 2 Ohm-cm² and amaximum of 10 Ohm-cm². More broadly, a membrane resistance of between0.5 and 20 Ohm-cm² could be considered.

For the iECP described herein, there is not a way to directly controlthe cell current density. One way to possibly control the hydrogenconsumption is by intentionally limiting the supply of hydrogen to thecell to produce a low average cell current through fuel starvation,although fuel starvation will provide a non-uniform current densitydistribution and poor CO₂ capture performance. Even if the cellresistances are properly tuned to give an optimal current density at oneflow rate of air, the HEMFC fuel cell system application requires thatthe flow rate to the iECP is increased and decreased as the flow rate tothe HEMFC is increased or decreased. If the flow rates are not increasedand decreased accordingly, too much hydrogen is consumed at a partialload.

Since the anode and cathode flow rates are the only parameters to becontrolled in the iECP, and the cathode flow rate matches the HEMFCload, the anode gas supply could be the target for controlling internalcurrent density.

To control the rate of hydrogen supply from the anode gas flow layer tothe anode, a diffusion barrier can be added to the anode that thenoperates at a diffusion-limiting current density determined by thebarrier. Normally at the iECP operating current density, mass transportis rapid and there is essentially no hydrogen concentration gradientbetween the anode gas flow layer and the anode electrocatalyst surface.Such a negligible hydrogen concentration gradient would not result is asignificant voltage loss and would not influence cell current density.

A way to control the cell current density for an iECP would be to put amicro porous or partially gas-permeable barrier between the anode andthe anode gas flow layer. Advantageously, such barrier would blockhydrogen transport, except for the small amount that could diffusethrough the barrier (e.g., on the order of 10 mA/cm²). As the cellapproaches this current density, the anode would run out of hydrogen andthe cell voltage would decrease to zero. The flux of hydrogen throughthe ionomer film and the limiting current density, are described by:

${N_{H2} = {\frac{i_{\lim}}{2F} = {\frac{D}{RT}\frac{p_{H2}}{L_{film}}}}},$where N_(H2) is flux of hydrogen, i is limiting current density, D isdiffusivity of hydrogen in the barrier, R is the gas constant, T istemperature, p_(H2) is the partial pressure of hydrogen, and L_(film) isthe thickness of the barrier. If we can control p_(H2), then we cancontrol i_(lim). The partial pressure of hydrogen can be controlled bychanging the total pressure, by recycling the CO₂-rich,hydrogen-depleted outlet gas, or by mixing in some air or HEMFC-exhaustair (e.g., less oxygen). The latter strategy would consume some hydrogenvia catalytic combustion, but would dilute the remaining hydrogen withnitrogen.

The diffusion barrier will cause CO₂ to build up to a higherconcentration in the anode. Here, it could be advantageous to use adiffusion barrier with selectivity for carbon dioxide over hydrogenpermeation, such as an ionomer film. Increasing carbon dioxidepermeation relative to hydrogen will minimize the carbon dioxidegradient from the anode to the anode gas flow layer. However, thesensitivity of hydrogen and carbon dioxide permeation rates totemperature and relative humidity must be considered as well. It wouldbe preferable to minimize this sensitivity to achieve more predictablecontrol of cell current density from hydrogen partial pressure.

The basic control method for the HEMFC and ECP described herein is toadjust the current density and hydrogen flow rates to be proportional tothe air flow rate demanded by the HEMFC. It may be advantageous toreduce current and hydrogen supply more than 1:1 with reducing airdemand, because the required ECP performance is lower as well, soadditional carbonate buildup is acceptable. This would reduce theparasitic hydrogen consumption when the HEMFC is at partial load.

For the iECP cell, the hydrogen recycle and hydrogen dilution strategiesare expected to only work with the hydrogen diffusion barrier. Thepulsed hydrogen flow is an alternative method that could work without ahydrogen diffusion barrier and has the advantage that most PEMFC systemimplementations use a pulsed purge, rather than a continuous purge. Theadvantages of this method probably apply to HEMFC systems as well.

If the cell is continuously starved of hydrogen, the result is a highcurrent density near the anode inlet and a very low current near theanode outlet, where hydrogen is depleted. If instead, hydrogen is pulsedat a high flow, the entire anode gas flow layer can be filled with ahigh concentration of hydrogen. At these conditions, the cell will go toits maximum design current density (e.g. 30 mA/cm²). Then, when thehydrogen supply is cut off, the hydrogen will be consumed uniformlyacross the entire cell from the anode gas flow layer. The current willstay at 30 mA/cm² until the hydrogen is depleted, and then the cellcurrent will quickly fall to zero. When the current reaches zero,carbonate will build up in the cathode and also start to diffuse overfrom the anode. The stored hydroxide will continue to capture CO₂ untilthe hydroxide is completely consumed. As long as the next hydrogen pulsecomes before the hydroxide concentration is too low, sufficient iECPperformance will be maintained. The current pulse will pump theaccumulated carbonate to the anode and replace it with hydroxide, andstart the cycle over again.

The ECPs described herein can be applied to carbon dioxide removal froma gas stream containing an electrochemically reducible component andcarbon dioxide into a gas stream containing an electrochemicallyoxidizable component. Possible cathode reactions include oxygenreduction, proton reduction (i.e., hydrogen evolution). Possible anodereactions include hydrogen oxidation, water oxidation (i.e., oxygenevolution), and ammonia oxidation.

The ECPs described herein can be used to remove an acid gas thatdissolves, reacts, or dissociates in water to form anions and protons,in whole or in part, from an acid gas-containing stream. These acidgases can include sulfur dioxide and hydrogen sulfide.

The ECPs described herein can be used to remove a basic gas thatdissolves, reacts, or dissociates in water to form cations andhydroxide, in whole or in part, from a basic gas-containing stream. Thebasic gases can include ammonia and organic amines. In this case, theanion exchange polymer is replaced with a cation exchange polymer, andthe gases to purify must be introduced to the anode. Hydrogen oxidation,ammonia oxidation, and water oxidation (i.e., oxygen evolution) can beincluded as anode reactions compatible with this cell. Oxygen reduction,proton reduction (i.e., hydrogen evolution) are a nonexhaustive list ofcathode reactions compatible with this cell.

Battery electrode reactions can be used in place of fuel cell reactionsfor the anode and cathode. In these cases, cyclic operation is required,with current flow and gas supply connections reversed periodically toalternate which electrode is the cathode and captures carbon dioxide,and which electrode is the anode and concentrates carbon dioxide.

Definitions

As used herein, the “cell pitch” is the shortest distance from theanode-membrane interface of one cell to the anode-membrane interface ofthe neighboring cell. Alternatively, it is the combined thickness ofanode, membrane, cathode, anode gas flow layer, cathode gas flow layer,and bipolar plate.

The “bipolar plate” is the part that separates adjacent cells in aseries-connected stack of cells and provides an electrical connectionbetween the cathode of one cell and the anode of an adjacent cell, whilekeeping the gas flow layers separate.

The “gas flow layer” is the layer of the cell through which gas flowsand from which gas may be exchanged with either the anode or the cathode(“anode gas flow layer” and “cathode gas flow layer”, respectively).

The “CO₂ mass transport resistance” is a performance metric of the ECPdefined as the average CO₂ concentration in the cathode gas transportlayer divided by the CO₂ removal rate per unit MEA area. Mathematically,the CO₂ mass transport resistance (R_(MT)) is calculated as

${R_{MT} = \frac{A}{v\left( {{\ln\left( x_{in} \right)} - {\ln\left( x_{out} \right)}} \right)}},$where A is the total MEA area in the ECP (units of m²), v is thevolumetric flow rate of CO₂-containing gas to the ECP (m³/s), and x_(in)and x_(out) are the CO₂ mole fractions in the CO₂-containing gas at theinlet and outlet of the ECP, respectively (unitless).

Having described the invention in detail, it will be apparent thatmodifications and variations are possible without departing from thescope of the invention defined in the appended claims.

EXAMPLES

The following non-limiting examples are provided to further illustratethe present invention.

Example 1: Modeling of the ECP for Removing CO₂

The mechanism for the ECP of carbon dioxide can be understood through aone dimensional membrane-electrode assembly (MEA) model incorporatingelectrochemical transport and reactions. The conversion between carbondioxide, bicarbonate, and carbonate is handled by assuming the water inthe ionomer behaves as a dilute aqueous electrolyte, using literaturetabulated rate constants and activation energy. The key reactions areCO₂+H₂O

H₂CO₃,  [11]CO₂+OH⁻

HCO₃ ⁻,  [12]Where in reaction [12] is dominant in the cathode and reaction [11] isdominant in the anode. Carbonic acid, bicarbonate, and carbonate caninterconvert according to two acid-base equilibria,H₂CO₃+OH⁻

HCO₃ ⁻H₂O,  [13]HCO₃ ⁻+OH⁻

CO₃ ²⁻+H₂O,  [14]

The net rate of CO₂ hydration is given by:

$\begin{matrix}{{r_{CO2} = {\epsilon_{ion}{\phi_{H\; 2O}\left\lbrack {{\left( {k_{1} + \frac{k_{2}c_{H}}{\phi_{H2O}}} \right)K_{H,{CO2}}p_{CO2}} - {\left( {\frac{k_{- 1}K_{b2}\phi_{H\; 2\; O}}{c_{H}} + k_{- 2}} \right)\frac{c_{B}}{\phi_{H2O}}}} \right\rbrack}}},} & \lbrack 15\rbrack\end{matrix}$where k₁, k⁻¹, k₂, k⁻² are the forward and backwards rate constants forthe neutral (eq. 11) and alkaline (eq. 12) CO₂ hydration mechanisms,respectively, ϵ_(ion) is the volume fraction of ionomer in theelectrode, ϕ_(H2O) is the volume fraction of water in the ionomer,K_(H,CO2) is the Henry's law constant of CO₂ in water, p_(CO2) is thepartial pressure of CO₂ in the gas pore, c_(i) is concentration of ioni, and K_(b2) is the acid-base equilibrium constant between carbonicacid and bicarbonate (eq. 13). The three key ions are designated bysubscripts H for hydroxide, C for carbonate, and B for bicarbonate.Electrochemical transport is modeled using the Nernst-Planck equation,

$\begin{matrix}{{N_{i} = {{- {D_{i}\left( \frac{dc_{i}}{dx} \right)}} - {\frac{z_{i}FD_{i}}{RT}{c_{i}\left( \frac{d\Phi_{2}}{dx} \right)}}}},} & \lbrack 16\rbrack\end{matrix}$where N_(i) is flux, D_(i) is diffusivity, z_(i) is charge, all of ioni. ϕ₂ is the ionic potential, x is the spatial coordinate. The simulatedconcentration profiles of hydroxide, carbonate, and bicarbonate areshown in FIG. 13 for an MEA with a 20 μm low-conductivity membrane (4Ω·cm²), 0.01 mgPt/cm² anode (5 wt % Pt/C), and 1 mg/cm² Ag cathode. Theone dimensional model was run with fixed flow channel composition as aboundary condition—in this case 400 ppm at cathode and 10,000 ppm atanode. The electric field generated at low current densities issufficient to maintain a pH gradient of about 6 units, which creates avery large difference in the anode and cathode equilibrium CO₂concentration, driving nearly irreversible CO₂ pumping.

The ability of the CO₂ ECP to achieve >99.9% removal of CO₂ from air isillustrated in FIG. 14 , which shows simulation results for a cathodeflow channel concentration of 0.4 ppm and an anode flow channelconcentration of 100,000 ppm. FIG. 14A shows anion concentrationprofiles, and FIG. 14B shows the CO₂ hydration/dehydration rate (e.g.capture/release respectively) at open circuit, 10, and 20 mA/cm². Atopen circuit, CO₂ is transported according to the concentrationgradient, but at 10-20 mA/cm², CO₂ is captured from the cathode.

Calculations estimate the characteristic length scale for CO₂reaction/diffusion into hydroxide form ionomer is only 50 nm at 70° C.Given this length scale, any CO₂ diffusing through the membrane from theanode towards cathode will react with hydroxide long before reaching thecathode.

Example 2: eECP Operating in Air/Hydrogen Mode

Proof-of-concept for the ECP was demonstrated experimentally using asingle Air/Hydrogen cell, to probe the effects of operating temperatureand current density on ECP performance in removing CO₂ from the airstream to the hydrogen stream. Cathode or anode outlet gases weremonitored by a CO₂ sensor (Vaisala GMP252). The first experiment used0.4 mgPt/cm² as 47 wt % Pt/C in both electrodes of a 5 cm² cell (#1) anddemonstrated CO₂ levels in the air exhaust below 100 ppm at low currentdensities (≤40 mA/cm²). Given this initial success and the tight costrequirements for the ultimate application, a 25 cm² cell (#2) wasprepared with low-cost electrodes: 0.013 mg_(Pt)/cm² as 5 wt. % Pt/C inthe anode and 0.6 mg/cm² of unsupported Ag in the cathode. The secondcell was investigated over a wider range of flow rates, demonstratingCO₂ removal to low ppm levels (determination limited by the accuracy ofthe CO₂ sensor). To demonstrate the room for performance improvement,cell #3 was fabricated using the same gas diffusion electrodes as cell#2, but with a porous carbon-ionomer interlayer applied directly to thecathode side of the membrane. Such interlayer provides more accessibleionomer volume for CO₂ reaction with hydroxide. All experiments used PAPmembranes and ionomers. The PAP membranes and ionomers are described inU.S. application Ser. No. 16/146,887, herein incorporated by reference.

When the cathode hydroxide concentration is sufficiently high, CO₂capture by cathode OH⁻ is expected to be a first-order, irreversibleprocess, and the CO₂ concentration is expected to decrease exponentiallydown the length of the cathode flow channel. Under these conditions,there should be a log-linear relationship between the outlet CO₂concentration and the inverse flow rate. Such a relationship means thatif we need 1 m² active ECP area to achieve 90% CO₂ removal, we canachieve 99% removal with 2 m² and 99.9% removal with 3 m². Thisfavorable characteristic calls for experimental evidence, which has beenprovided.

FIG. 15 describes the CO₂ removal capacity of a single air/hydrogen cellof 25 cm² with serpentine flow fields (Cell #2), serving as an ECP. TheCO₂ level in the air exhaust was measured as a function of air flow rateat temperatures between 50° C. and 70° C. The results showed that, atlow flow rates, it is possible to achieve CO₂ removal down tosingle-digit ppm CO₂. Based on the anode flow rate of 50 sccm, the anodeoutlet CO₂ concentration should range from 700 to 3000 ppm, showing thatCO₂ could be pumped against an approximately 3 orders of magnitudeconcentration gradient with no loss of performance. Except in caseswhere flooding was suspected, the CO₂ pump shows first-orderirreversible behavior up to 99% CO₂ removal, where the limits of sensoraccuracy were reached.

FIG. 16 shows CO₂ removal and calculated CO₂ mass transport resistanceat 70° C. for Cells #2 and #3 and at 80° C. for Cell #1, all at 20mA/cm². Cell #2 had lower performance than Cell #1, which is likely dueto lower ionomer loadings in the cathode, which limited the reactionwith hydroxide. Cell #3 showed the best performance, with half the masstransport resistance compared to Cell #2. Cell #3 used a multilayercathode structure that incorporated more ionomer volume for CO₂ capturewithout using a thick electrocatalyst layer. A thinner electrocatalystlayer would be particularly advantageous if the electrocatalyst isexpensive. The mass transport specific resistance is nearly constantwith CO₂ concentration (FIG. 16 b ), indicating an ideal first-orderprocess. Under these conditions, moving from 90 to 99.9% CO₂ removalrequires only tripling of membrane area, making it possible to achievethe air purity specification for the HEMFC stack.

Example 3: iECP Operating in Air/Hydrogen Mode

Additionally, the iECP concept aimed to achieve simplest operation of,likely, the least expensive ECP, was demonstrated experimentally. A PAPmembrane was cast with 30 wt % carbon nanotubes to create internalelectronic short, and was made into a MEA using 0.4 mgPt/cm² in the Pt/Ccatalyzed electrodes. The 5 cm² cell was assembled and tested in a rangeof cell temperatures, with hydrogen or nitrogen on the anode side, and350 ppm CO₂-containing air on the cathode side. The results are shown inFIG. 17 and roughly match the performance of a similar non-shorted 5 cm²MEA. Due to the low overall cell area, ultra-low levels of CO₂ in theair outlet were not achieved, but similar mass transport coefficientswere calculated compared to the cells using non-shorted MEAs.

When introducing elements of the present invention or the preferredembodiments(s) thereof, the articles “a”, “an”, “the” and “said” areintended to mean that there are one or more of the elements. The terms“comprising”, “including” and “having” are intended to be inclusive andmean that there may be additional elements other than the listedelements.

In view of the above, it will be seen that the several objects of theinvention are achieved and other advantageous results attained.

As various changes could be made in the above devices and methodswithout departing from the scope of the invention, it is intended thatall matter contained in the above description and shown in theaccompanying drawings shall be interpreted as illustrative and not in alimiting sense.

What is claimed is:
 1. An electrochemical pump (ECP) for separatingcarbon dioxide from a carbon dioxide-containing gas comprising: a cell,the cell comprising: two electrodes that are capable of serving as ananode or a cathode, the two electrodes each independently comprising acharge-storage compound and an anion exchange polymer, the chargestorage compound being capable of reacting to form hydroxide ions whenserving as cathode and reacting to consume hydroxide ions or produceprotons when serving as anode; and a membrane adjacent to and separatingthe two electrodes; wherein the cell is adapted such that in operation:the carbon dioxide-containing gas is contacted with the electrodeserving as cathode and the carbon dioxide reacts with the hydroxide ionsto form bicarbonate ions, carbonate ions, or bicarbonate and carbonateions; the bicarbonate ions, carbonate ions, or bicarbonate and carbonateions are transported to the electrode serving as anode through themembrane; and the bicarbonate ions, carbonate ions, or bicarbonate andcarbonate ions react at the electrode serving as anode to form carbondioxide and water.
 2. The ECP of claim 1, wherein one or both of the twoelectrodes comprise a metal oxide, a metal hydroxide, a metaloxyhydroxide, or a hydrogen storage alloy as the charge-storagecompound.
 3. The ECP of claim 2, wherein the metal oxyhydroxidecomprises nickel oxyhydroxide.
 4. The ECP of claim 2, wherein the metalhydroxide comprises nickel hydroxide.
 5. The ECP of claim 2, wherein themetal oxide comprises manganese dioxide.
 6. The ECP of claim 2, whereinthe hydrogen storage alloy comprises a lanthanum nickel hydride.
 7. TheECP of claim 1, wherein the membrane comprises an anion exchangepolymer.
 8. The ECP of claim 1, further comprising a power supply forsupplying a current flow to the electrodes, wherein the power supply isadapted to alternately reverse direction of current flow, therebyallowing each of the electrodes to serve, in turn, as the anode and asthe cathode.
 9. The ECP of claim 1, wherein the cell further comprisesporous ionomer layer between the membrane and each of the twoelectrodes.
 10. The ECP of claim 9, wherein the porous ionomer layercomprises an anion exchange polymer.
 11. The ECP of claim 9, wherein theanion exchange polymer of the two electrodes, the anion exchange polymerof the membrane and/or the anion exchange membrane of the ionomer layerindependently comprise quaternary ammonium or imidazolium groups and apolymer backbone not having ether groups.
 12. The ECP of claim 9,wherein the anion exchange polymer of the two electrodes, the anionexchange polymer of the membrane and/or the anion exchange membrane ofthe ionomer layer independently comprise poly(arylpiperidinium),alkylammonium-functionalized poly(aryl alkylene),substituted-imidazolium-functionalized poly(aryl alkylene),alkylammonium-functionalized poly(styrene),substituted-imidazolium-functionalized poly(styrene),alkylammonium-functionalized poly(styrene-co-divinylbenzene),substituted-imidazolium-functionalized poly(styrene-co-divinylbenzene),alkylammonium-functionalizedpoly(styrene-block-ethylene-co-butadiene-block-styrene),substituted-imidazolium-functionalized,poly(styrene-block-ethylene-co-butadiene-block-styrene),alkylammonium-functionalized poly(ethylene),substituted-imidazolium-functionalized poly(ethylene),alkylammonium-functionalized poly(tetrafluoroethylene),substituted-imidazolium-functionalized poly(tetrafluoroethylene),alkylammonium-functionalized poly(ethylene-co-tetrafluoroethylene),substituted-imidazolium-functionalizedpoly(ethylene-co-tetrafluoroethylene), polyethyleneimine, poly(diallylammonium), polydiallyldimethylammonium or a combination thereof.
 13. TheECP of claim 9, wherein the anion exchange polymer of the twoelectrodes, the anion exchange polymer of the membrane and/or the anionexchange membrane of the ionomer layer independently comprise poly(arylpiperidinium) or poly(diallyl ammonium), or polydiallyldimethylammonium.14. The ECP of claim 1, wherein the carbon dioxide-containing gas isair.
 15. A system comprising: a metal-air battery; and theelectrochemical pump (ECP) of claim 1; wherein the ECP is adapted suchthat in operation, the carbon dioxide-containing gas is air and afterthe air is supplied to the ECP to reduce the concentration of the carbondioxide, the air having the reduced concentration of carbon dioxide isdirected to a cathode inlet of the metal-air battery.
 16. A batterysystem comprising: a metal-air battery; and an electrochemical pump(ECP) for separating carbon dioxide from a carbon dioxide-containinggas, the ECP comprising: a cell, the cell comprising: a cathodecomprising a cathode electrocatalyst for reducing oxygen to formhydroxide ions; an anode comprising an anode electrocatalyst foroxidizing a reagent to form protons or consume the hydroxide ions; and amembrane adjacent to and separating the anode and the cathode; whereinthe ECP is adapted such that in operation with air as the carbondioxide-containing gas: the carbon dioxide-containing gas is supplied tothe cathode and the carbon dioxide reacts with the hydroxide ions formedat the cathode to form bicarbonate ions, carbonate ions, or bicarbonateand carbonate ions; the bicarbonate ions, carbonate ions, or bicarbonateand carbonate ions are transported to the anode through the membrane;the bicarbonate ions, carbonate ions, or bicarbonate and carbonate ionsreact at the anode to form carbon dioxide and water; and after the airpasses through the cathode of the ECP to reduce the concentration of thecarbon dioxide, the air having the reduced concentration of carbondioxide is directed to a cathode inlet of the metal-air battery; whereinat least one of the following: (a) the anode and the cathode areelectronically connected through the membrane to form an internalcurrent ECP (iECP), and the membrane comprises an anion exchange polymerand an electronically-conductive material or anelectronically-conductive anion exchange polymer; or (b) a porousstructure-ionomer interlayer separates the membrane from the cathode.17. A method for separating carbon dioxide from a carbondioxide-containing gas comprising supplying the electrode serving as thecathode of the electrochemical pump (ECP) of claim 1 with the carbondioxide-containing gas.
 18. The method of claim 17, wherein the carbondioxide-containing gas is a flue gas.
 19. The method of claim 17,wherein the carbon dioxide-containing gas is air.
 20. The method ofclaim 17, wherein electrical current is driven through the ECP in afirst phase in which one of the two electrodes serves as the anode andthe other of the two electrodes serves as the cathode and a second phasein which current is driven so that the one of the two electrodes servesas the cathode and the other of the two electrodes serves as the anode.